Servo is in the design phase, some code has been written, but large parts are missing. It's not going to become a viable project any time soon; however, the team is working on the premise that Servo may eventually be used in an actual browser.

It's going to be years before it can replace Gecko, if that happens at all, but it's good to see that Mozilla is at least entertaining the idea of a Gecko replacement.

Gecko is no slouch, but it along with Firefox's existing codebase has prevented Mozilla from developing a proper multi-process architecture for the browser.

The Electrolysis project has been put on hold after a couple of years of work didn't provide much tangible results. Firefox only separates the plugin process from the main one.

Chrome runs each tab, plugin, extension and so on in separate processes. This is done at the browser level, WebKit, the rendering engine in Chrome, is still single-process. WebKit2, a proposed successor, is designed with multiple processes in mind.